A major requirement of modern burners, especially of burners used as part of a gas turbine, is to cover a greatest possible power range with the lowest emissions possible. The undesired emissions concerned are in particular carbon monoxide emissions (CO emissions) and nitric oxide emissions (NOx emissions). Basically the power of a burner is almost proportional to the flame temperature and to the air mass flow. Operation at low power means a low flame temperature, whereby CO emissions increase markedly. In addition the flame also becomes longer in such cases, which with cooled burner walls leads to quench effects, also resulting in increased CO emissions.
With a gas turbine the result can also be thermo acoustic instability over the entire operating range, which can jeopardize safe operation of the combustion system. Such thermo acoustic instability is frequently also referred to as “vibration” and can occur especially with the premix burners currently generally used.
As a rule the burners of a gas turbine must be switched off below a critical temperature limit at which the flame becomes unstable or the CO emissions become too high. If necessary other burner stages must be operated, as a rule diffusion burners, which however then create high NOx emissions.